There has been known a treatment method in which an affected part of a patient suffering from cancer and the like is irradiated with a charged particle beam (ion beam) such as a proton and carbon ion. A charged particle beam irradiation system (particle beam extraction system or charged particle beam extraction device) used in this treatment is provided with a charged particle beam generator, and the ion beam accelerated in the charged particle beam generator reaches an irradiation device installed in a rotating gantry by passing through a first beam transport line and a second beam transport line provided in the rotating gantry. The ion beam is extracted from the irradiation device, and the affected part of the patient is then irradiated with the ion beam. As the charged particle beam generator, for example, there is known a synchrotron (circular accelerator) provided with a unit for causing a charged particle beam to revolve along an orbit, a unit for bringing a betatron oscillation of the charged particle beam into a resonance state outside a stability limit of resonance, and an extraction deflector for taking out the charged particle beam from the orbit (see, for example, PTL 1).
The irradiation device shapes the ion beam lead from the above-described ion beam generator according to the depth of the affected part from a body surface of the patient and a shape of the affected part, and then irradiates the affected part of the patient on a treatment couch with the ion beam. In general, an irradiation device irradiates an affected part with an ion beam by using any of the beam irradiation methods including the double scattering irradiation method (NPTL 1, page 2081, FIG. 35), the Wobbler method (NPTL 1, page, 2084, FIG. 41), and the beam scanning irradiation method (PTL 2, and NPTL 1, pages 2092 and 2093).
Among the above-described beam irradiation methods, the beam scanning irradiation method is drawing attention as it has less influence on normal cells and requires no device incorporating a nozzle. The beam scanning irradiation method is characterized by irradiation of a beam according to a shape of the affected part by combining scanning in a direction perpendicular to a beam travelling direction by controlling a current amount for exciting a scanning magnet, and scanning in a depth direction of the affected part by changing energy.
In the beam scanning irradiation method, each element constituting an irradiation region is referred to as a spot, and an affected part divided into multiple regions in the depth direction is referred to as a layer. A current value the scanning magnet is set, and a set dose of irradiation is performed when reaching a target spot. Once the irradiated dose reaches a set value, an irradiation position is moved to the next spot. During a move from a spot to the next, extraction/of the beam is stopped. Once the move of the irradiation position is completed, the beam is extracted again. This is repeated until the irradiation is completed within one layer. Once the irradiation within one layer is completed, a similar irradiation is repeated by changing to energy for the next layer. This is repeated until the irradiation is completed in all layers. This is an outline of the beam scanning irradiation method.
When a charged particle beam is injected into a material (a body of a patient), it has a physical property to release most of the kinetic energy thereof just before stopping, and to form a dose distribution having a maximum called the Bragg peak. By adjusting the energy of the charged particle beam and by substantially aligning the Bragg peak with a position of the affected part in the depth direction, it is possible to give an intensive dose to the affected part.
A multi-layer beam monitor is an example of a unit for measuring the dose distribution. The multi-layer beam monitor has a dose measuring unit constituted by layering a plurality of ion chambers. The multi-layer beam monitor can measure the dose distribution inside the multi-layer beam monitor accurately and promptly in one irradiation (PTL 3). In general, the multi-layer beam monitor is disposed in a position downstream of the irradiation device where the patient is originally to be disposed. Prior to actual irradiation of the patient, by measuring the dose distribution by irradiating the multi-layer beam monitor, it is possible to check whether or not the predetermined energy, Bragg peak, and irradiation depth are realized.
Now, as a technology for efficiently irradiating an affected part with a beam, multi-energy extraction is drawing attention (PTL 4). In general, one cycle of a charged particle beam irradiation system includes injection-acceleration-extraction-deceleration, and a beam is extracted in multiple cycles while changing the energy. Meanwhile, in the multi-energy extraction, irradiation is performed in multiple different energies in one cycle such as injection-acceleration-extraction-acceleration-extraction-(repeated)-extraction-deceleration, or injection-acceleration-extraction-acceleration-extraction-(repeated)-extraction-deceleration-extraction-deceleration-extraction-(repeated)-extraction-decelerate. By using the multi-energy extraction, irradiation of the whole affected part can be completed in one cycle of operation of the synchrotron.
On the other hand, in the charged particle irradiation system, among the charged particle beams extracted from the charged particle beam generator, there may be an unnecessary charged particle beam not transported to the irradiation device. In order to process such unnecessary beam, it has been considered to provide a beam damper device in a first beam transport line (PTL 5).